The present invention relates to a method of forming a Josephson junction, particularly a weak-link junction, of oxide superconductor, and also to a superconducting device employing such a junction.
Owing to the advent of oxide superconductors, the critical temperatures (Tc) of which exceed the boiling point of liquid nitrogen (77 K), new superconducting devices are expected. However, when a device, especially an electronics device, is to be formed using an oxide superconductor, several problems are left unsolved. One of them is the formation of a Josephson junction. In a conventional Josephson junction device employing a metal superconductor, a tunnel junction in which a very thin insulating layer is interposed between thin superconductor films has been formed. The main reasons for the realization of this junction are that, in the case of the metal superconductor, an insulating film (of, for example, SiO.sub.2) which does not react with the thin superconductor films and which exhibits an excellent insulation even with a very small thickness can be formed, and that the metal superconductor has a coherent length as great as several tens of .ANG.. Superconducting switching devices, etc. have been developed by employing such tunnel junctions.
With the high-temperature oxide superconductor, however, (1) since the coherent length thereof is as small as several .ANG., the insulating layer must be made thinner than in the conventional case, and (2) in order to obtain thin superconductor films of high Tc, the films must be formed under the condition of a high substrate temperature or must be annealed at high temperatures. Furthermore, unlike the metal superconductors, the oxide superconductor is prone to react with insulating materials (in general, oxides). Therefore it is very difficult to form the tunneling type Josephson junction. For these reasons, Josephson junctions have heretofore been obtained only when a bar 10 of superconductor ceramics is pressed against the superconductor 11 as shown in FIG. 14, and a Josephson junction in the form of layered films has not been fabricated.
As one Josephson junction fabricated of a thin film, there has been known as a weak-link junction in which a superconducting link is weakened by thinning the film partly or by narrowing a portion for the flow of current.
A practicable method of forming the weak link is one wherein, as shown in FIG. 15, a substrate 20 is provided with a step 21, and the thickness of the thin superconducting film 22 formed thereon is changed. In an alternative method hydrogen ions are implanted, thereby causing the superconducting current to flow through only the narrow region. In another method wherein grain boundaries are utilized, and so forth. Examples of the method which utilizes the grain boundaries are described in Phys. Rev. Lett. 60 (1988) 1653 and the official gazette of Japanese Patent Application Laid-open No. 273782/1987. With these examples, however, it is expected that the grain boundaries of the polycrystalline superconducting thin-film will happen to come to a place where the junction is to be formed. Besides an inferior reproducibility, the examples have had the problem that the shape and position of the junction cannot be selected at will. Further, they have had the problem that, since a plurality of grain boundaries exist, a large number of junctions are formed in series in the direction of the current, resulting in an inferior I-V characteristic.
In the technique of Japanese Patent Application Laid-open No. 273782/1987, epitaxial growth is applied for the purpose of reproducibly forming weak-link Josephson junctions of uniform characteristics at a predetermined position. According to the prior-art technique, a slit-like pattern is formed on a single-crystal substrate, and a thin superconducting film is epitaxially grown thereon. The thin superconducting film on the slit-like pattern is hindered in the epitaxial growth, to become polycrystalline at the beginning. The thickness of the film increases later, with the result that a single grain boundary is formed on the slit-like pattern. With the prior-art method, however, a photoresist process is required for forming the pattern, so that the contamination of the surface of the substrate, which is a factor for hampering the epitaxial growth, is liable to occur and degrades the reproducibility. Another problem has been that, since the pattern has its minimum width limited by microfabrication technology, it is difficult to produce a thin superconductive film.